Type IV pili are complex surface organelles that are located on one or both cell poles of many bacterial cells. Genetic, biochemical, and behavioral studies have demonstrated that these structures are important for motility, DNA uptake during transformation and conjugation, biofilm formation, pathogenicity, and cell-to-cell communication. One of the model organisms to study the structure and function of type IV pili is Myxococcus xanthus. In this bacterium type IV pili generate the propulsive force for social- (S)-motility, a specialized form of motility that is used by many gram-negative bacteria, when in contact with a solid surface. We have recently isolated a novel pilus from M. xanthus that is distinct from the well-characterized type IV pilus involved in S-motility (PilA, 23 kD, MXAN_5783). This novel pilus is composed of a 16 kD protein termed ParP (MXAN_7475) that is located within the operon encoding the chromosomal partitioning proteins ParA and ParB suggesting a link between pilus expression and/or function and cell division. Sequence analysis suggests that ParP shares similarities with type IVb pilins and may be part of an additional functional S-motor. This interpretation is further supported by three important observations: First, cells expressing the ParP pilus but lacking both adventurous- (A)-motility and PilA-based S-motility can spread as coherent groups of cells in an S-like fashion indicating that the pilus represents a novel molecular motor; second strains carrying a parP knockout are unable to aggregate and form fruiting bodies and show reduced S-motility in swarming assays; all three features are hallmarks of S-motility. To study the role of the ParP pilus in the complex life cycle and coordinated behaviors of myxobacteria, the following specific aims will be undertaken: 1) Characterize ParP, the polymerized pilus, and identify genes required for its biogenesis. 2) Define the role and occurrence of ParP pili. 3) Study the transcriptional expression of ParP, A, and B as well as possible interactions between these three proteins. PUBLIC HEALTH RELEVANCE Pili are filamentous bacterial surface structures that are important determinants of bacterial pathogenicity and play a major role during infectious processes. A better understanding of the structure, function, and assembly of these structures in a model organism may result in the development of new strategies to fight bacterial infections.